Hi all,

This is a subject that I have much interest in. Not wanting to hijack the thread that this wad posted in, I decided to crate a new one.

Most people know I have been involved with cutting grooves in squish pads in attempt to enhance end gas combustion, that’s not what this is about.

I’m interested in learning more about what Denis calls a properly designed quench region. How does one determine what is needed to properly control the charge front? What is needed to assure the eng-gas burn rate is sufficient?

My work is primarily on SBC engines, generation I, II & III. In addition I do an occasional BBC, 4.250” bore based.

 

A-B

The way I describe the squish-quench is the the squish is on the opposit side of the spark plug where there is the most area to push the A/F mixture towards the plug. When both sides are equal then the plug has a hard time igniting the AFM.

What I generally do is to eliminate the shelf on the intake side of the chamber and reduce the quench side as much as possible. Then I look at the plug to be sure that it is entirely exposed to the the volitile mixture inside the chamber area. If I get too extravigent with that I have to mill the head back the some cc so that compression can be right for the purpose we are using it for. If possible we try to use flat top pistons, zero dome, or no more than .150 dome hight.

If we can't accomplish our C/R in that manner we have to do some serious work on the dome to get the flame front to cross over the top of the piston as it nears TDC.

As I see it this is where your grooves come in to play, by forcing the volitile mixture directly into the flame front as it expands outwards away from the plug. Since a lot of this mixture gets trapped on the squish area and doesn't get burned correctly.

Another thing we have found is that the closer the piston can come to the head-deck the better off you are. Now you can get by with .025 clearance so long as the engine remains below 7200 RPM - what if we can keep it below 5000 how close could it be then??? Chevy used to make .017 embossed stainless steel head gaskets - I wonder how that would work on a street only engine that never see's 5000 RPM???

Denny

 

Denny,
I’m surprised that you are reducing squish area. With the exception being the concave dished pistons being used in cup engines, most people recommend a large squish to bore ratio. On the other hand, the LSX head fits your description and works very well; but then Dart’s version of the same has a squish area on the plug side.

I’d like to here the theory behind what you are doing and the benefits realized. Can you post pictures? I have a 468 BBC with a set of Dart pro1 heads that I’m building for bracket racing. It would be a good candidate for what you describe.

Most people I talk to are recommending reducing the squish clearance to a point where the piston almost touches the head at RPM, so this is not a surprise. A small squish pad set up where the piston nearly touches the head leaves little mixture in the end zone and produces maximum squish velocity, this is fully understandable. I do hear people that run low RPM are running very tight squish clearances like you describe.

 

I have no practicle knowledge on the subject but thanks to the internet I have done quite a bit of internet reasearch on the subject. From what I have gathered companies are tending to increase the quench surface area and reduce the combustion chamber size. Like was said by automotivebreath increasing the flat surface behind the spark plug.

From what I have read you would want to keep the combustion area as small as practicle and with the least amount of obstruction for the flame travel. I have read that most do not like to have a dome any higher than .100" with some saying .150". With higher domes some have cut a groove through the dome to enhance flame travel.


I find this a very interesting subject. I was hoping more people "in the know" would have jumped on this thread. Maybe another one of those "racing secret" things like the use of low tension rings for street use.

 

Here's a couple of places to get some further info - one has pic's:

www.wighat.com/fcr3/

ENDYN www.theoldone.com

I reduce the quench area behind the plug to keep the pressure from that side less than the main flat side. The front side pushes the end-gas towards the plug and the expanding flame-front. If you can shape the back side correctly then its pressure wave forces the A/F/M up at the plug.

The secondary problem is the liquid being forced into the chamber area from the intake valve. It tends to swirl around and gather over by the exhaust valve, and then as the piston gets close to TDC it moves its pattern closer to the center of the chamber, flooding the plug with still liquid A/F/M.

It is this specific reason that the Cup engines are using concave pistons, because that tends to allieviate that problem.

Denny

 

Like you I would like to hear different opinions on this. Looking at the countless combustion chamber designs of the 23 degree SBC engine; no two head manufacturers use the same chamber design. I have read that a compact combustion chamber design is desirable and currently favor the LT1 or RHS/Topline chamber, very compact and plenty of squish area.

I’m not trying to disagree with Denny, more trying to understand his rational of reducing plug side squish area in favor of enhancing squish flow from the far side. Denny, here's a picture of a set of LT1 heads I'm porting, how would you go about modifying this chamber? Drag racing application, 7500 RPM with flat top pistons. Airflowdevelop, I’d love to hear your thoughts.

 

That looks like an aluminum head that's been severly milled

Ok the front side is ok - but look at the shelf above the plug - here the plug is partially hidden - it will give you more definition if you install the plug. I machine the chamber walls around the valves out to the gasket dia. Scribe the bore dia around the back side. Make a gage the is parallel with the deck and the plug angle then taper the back wall according to the plug angle. You can give it more angle if you wish and can still maintain the C/R you desire.

The big problem with all 23 heads is getting high C/R with a low dome volume.

If you are building a purely race engine shoot for 13.5 or more with alum heads and 12.5 with iron heads, depending on your wallet since some racing gas costs have risen to $16.00/gal.

If you check out that ENDYN site you will see some of the things he did to create The Soft Head - meaning soft to the gas reducing detonation. Plus he has a pulse generator on his flow bench so he can simulate the cam/piston action. Larry Widmer is a very sharp guy.

Denny

 

Soft heads = unforgiving heads. Not just detonation but to timing and air fuel adjustments, you could run very retarded timing and lean A/F without detonation problems and significant losses in power. VERY good for drag strip tuning with digitial ignition systems. Actually "Soft Head" wasn't Windmers term at all but a old Chyrsler engineer that he knew well coined that phrase.

FWIW that head is not severelly milled, it's probablly 54cc LT1 chambers which aren't that far from stock ported ones. At the most we are talking .030" looking at that picture. I've seen them angle milled and the little eyebrow by the exhaust/plug being much smaller than that.

Bret

 

Not being an expert with cylinder heads by any means, I'm going to take a guess at something here....

I bet the reason you want all your quench on the far side of the chamber is becuause that's the mixture you need to move towards the plug as much as possible to get it combusted. Stuff that's already near the plug doesn't need the help because it's already right next to the plug anyway.

 

Here’s a picture with the plug installed and a line drawn at the bore diameter. I think I understand what you are saying, cut the plug side squish pad on the angle of the plug so that it is tapered downward as it gets further from the gasket area.

Denny, what benefits are you realizing from this modification?

Bret’s right, these heads are not severely milled; they had a 0.030” flat cut. The combustion chambers measure 52 cc’s after the mill and valve area shroud work. With a chamber this small and attention to piston valve reliefs, deck height and gasket thickness decent compression can be had with flat top pistons and a 3.48” stroke.

Bret, what modifications do you recommend to this style chamber?

 

Nothing easy.... the goal of a good combustion space is that you want to move the air/fuel into the exhaust side of the chamber/piston next to the plug and light it off as late as you can.... cutting the quench pads unevenly and at a angle IS VERY hard to do. Better off fixing the pistons.

Bret

 

Brett, I more concerned with the magnitude of the benefits than the degree of difficulty. I can see the benefit of a small dome on the piston in the plug side squish area matched to a concave squish region in the head. The result would be squish on two different planes.

But that’s not what he is describing; a concave area on the head and a flat top piston would create a squish area that is tight at the gasket and wide at the plug. The question remains, what’s the benefit?

 

The goal of putting as much as you can in a small sphere shape with the plug in the center of it.

But aren't most combustion chambers "a concave area on the head and a flat top piston would create a squish area that is tight at the gasket and wide at the plug."

Bret

 

Denny's suggesting eliminating the plug side squish area and making it part of the chamber cavity.

 

WTF would you want to do that for?

I didn't read that he said to elminate it????? Maybe make it a few thou larger than the valley side of the quench area.

Bret